Lightgcn fix (#602)

* Add git ignore

* Lightgcn fix

Removed normalization for layers, not used for lgcn.
Fixed sum weight constant to num layers instead of cur layer index.
Allow lgcn to take blocks.
Fixed requirement error caused by newer dgl versions.
Moved edge normalization to graph for easier use.

* Lightgcn debug error fix

* Simplified layer normalization and readability

* Easier support of rcuda